It is known to incorporate radar-absorbing material (RAM) into composite structures such as wind turbine blades. This is done to reduce the radar reflectivity of the blades so that they do not interfere with radar systems such as air traffic control systems or marine radar systems. The frequency range of these radar signals is approximately 1-10 GHz, and hence the RAM incorporated in wind turbine blades is typically optimised to attenuate radar signals in this frequency range.
Many radar-absorbing materials are based upon the Salisbury Screen, which comprises three layers: a lossless dielectric layer sandwiched between a reflector layer or ‘ground plane’ and an impedance layer or ‘lossy screen’. The lossless dielectric is of a precise thickness equal to a quarter of the wavelength of the radar wave to be absorbed; the ground plane comprises a layer of highly reflective conductive material such as metal or carbon; and the lossy screen is generally a thin resistive layer.
Circuit analogue (CA) RAM technology has proven to be particularly effective for use in wind turbine blades. This is similar to the Salisbury Screen arrangement, but the impedance layer is a CA layer comprising an array of elements, such as monopoles, dipoles, loops, patches or other geometries. The CA layer and the ground plane form a radar-absorbing circuit in the composite structure. The RAM employed in modern wind turbine blades typically uses a thin layer of carbon tissue, also referred to as ‘carbon veil’, as the ground plane.
Experimental tests have shown that the conductive ground plane employed in RAM has the potential to interfere with lightning protection systems, such as those incorporated in wind turbine blades to protect the blades from damage caused by lightning strike events. To illustrate this problem, a typical lightning protection system of a wind turbine blade will now be described with reference to FIGS. 1a-1d. 
FIG. 1a is a plan view of a tip end 10 of a wind turbine blade 12. A lightning receptor 13 comprising a metal disc 14 is located on a suction surface 16 of the blade 12, near the tip 18 of the blade 12. Referring to FIG. 1b, which is a cross-sectional side view through the tip end 10 of the blade 12, it can be seen that the metal disc 14 is the head of a bolt 20. The bolt 20 is screwed into a conductive base 22, which is implanted within the tip end 10 of the blade 12. A similar bolt 24 is screwed into the opposite side of the base 22 to define a lightning receptor 26 on a pressure side 28 of the blade 12, at the tip end 10.
Referring still to FIG. 1b, the base 22 is connected to a lightning cable 30 via a connector element 32. The lightning cable 30 is earthed and extends longitudinally inside the blade 12, in a span wise direction, to the blade root. The cable 30 is surrounded by an insulating sheath and is attached to the main spar 32 of the blade 12 to prevent potentially damaging flashover discharges to the spar 32 from occurring.
In addition to the lightning receptors 13, 26 at the tip end 10, a series of secondary receptors 34 (FIG. 1c) are provided at intervals along the length of the blade 12. Referring now to FIG. 1c, which is a cross-section through an aerofoil part of the blade 12, between a leading edge 36 and a trailing edge 38, the secondary receptors 34 are also in the form of metal bolts 40 (FIG. 1d), which are screwed into respective receptor bases 42 (FIG. 1d) located adjacent an inner surface 44 of the blade shell 46. The secondary receptors 34 are connected to the lightning cable 30 (FIG. 1c) via connecting straps 48 extending between the base 42 of the respective receptor 34 and the lightning cable 30.
The lightning receptors 13, 26, 34 are designed to attract lightning strikes and channel electricity safely to ground via the lightning cable 30. Lightning clouds induce an electric field around the lightning receptors 13, 26, 34. The induced electric field is a low-frequency electric field, typically of the order of 10 MHz and below.
Referring to FIG. 1d, which is an enlarged view of the circled part 50 of FIG. 1c, a CA layer 52 is embedded within the composite structure of the blade shell 46, at a location between the inner surface 44 of the shell 46 and an outer surface 54 of the shell 46. A continuous carbon reflector layer 56, which serves as the ground plane, is adhered to the inner surface 44 of the shell 46. The lightning receptor 34 penetrates both the CA layer 52 and the carbon ground plane 56.
As shown in FIG. 1d, the base 42 of the lightning receptor 34 is close to, and in fact is in contact with, the conductive carbon ground plane 56. In this arrangement, the conductive carbon ground plane 56 tends to distort and reduce the induced electric field around the lightning receptors 13, 26, 34 in the presence of a charged lightning cloud. This can degrade the performance of the lightning receptors 13, 26, 34. Also, the conductive carbon ground plane 56 may be at a low potential, which presents a risk of potentially damaging flashover discharges occurring between the lightning cable 30 and the ground plane 56, or even in extreme cases, lightning striking the ground plane 56 in preference to the lightning receptors 13, 26, 34.
Against this background, it is an object of the present invention to provide RAM that is more compatible with lightning protection systems.